Broadband planar array antennas have become increasingly more important for both military and commercial applications. The broadband requirement is driven by the proliferation of wireless systems operating at different separate frequencies and the need for high speed. The planar form factor is desirable and often necessary for both transport and installation of the array antenna, because of the associated features of low profile and conformability to platform. The planar form also lends itself to low weight and low-cost production methods such as a printed-circuit board.
The planar array antenna consists of identical and generally equally-spaced element antennas periodically positioned on a planar surface (the x-y plane) of the array antenna, as depicted in FIG. 1. The periodicity is along two generally oblique coordinates, s1 and s2, which allows us to divide the plane into similar unit cells, with the center cell abcd shown in the figure. Although the arrangement of FIG. 1 shows only 9 unit cells, an infinite number of cells are implied here. (Note that while a real phased array must be finite in size, in theory an array of infinite extent is often assumed. The infinite planar array model greatly simplifies the theoretical problem, and has been well established since its introduction four decades ago.)
The array elements are fed by the feed and beam steering network, as depicted in the cross-sectional view of the array in FIG. 2, to generate a selected amplitude and phase distribution in the array elements so that they form a main beam in a desired direction. The antenna beam is scanned or steered by variation of the phase of the elements by means of their phase shifters; thus the antenna is called a phased array. Although the discussion herein is for the case of transmit, by reciprocity it applies also to the case of receive.
Although the possibility of broadband planar beam-scan array had been envisioned four decades ago (Wheeler, 1965), the design of broadband planar arrays has been mostly focused on arrays using 3-dimensional (3-D) elements such as the flared slots. 3-D elements have a large dimension perpendicular to the plane of the array (along the z axis), thus are not amenable to many low-cost production techniques. As a result, research efforts have been launched since late 1990s to explore the use of 2-dimensional (2-D) array elements, such as planar patches, flat dipoles, and slots, in planar arrays. Findings so far have shown that planar arrays of 2-D elements have the potential of wide bandwidths, large scan angle, as well as reduced thickness and weight. Since planar beam-scan arrays with 2-D elements are amenable to truly low-cost printed-circuit-board production, their potential applications in the commercial and military markets are recognized.
Hansen (1999) showed that a planar phased array using planar dipoles, without a ground plane, exhibits easy-to-match active resistance and fairly stable element gain pattern, over a wide range of scan angles and bandwidth (over 5:1). Yet the reactance remains to be matched over the frequency. Also, since this array does not have a ground plane, it has a bidirectional radiation pattern (on both sides of the array plane). The resulting bidirectional radiation renders this planar array unsuitable for applications in which conformal mounting on a platform is required. When Hansen added a ground plane to one side of array to suppress its back radiation, he noted disruptive effects. Therefore, Hansen's array is impractical, just like Wheeler's array, until a ground plane is added.
Following Hansen's reporting, research efforts in planar arrays soon escalated, essentially following two approaches: the Current Sheet Antenna (CSA) and the Fragmented Aperture (FA).
The CSA approach was taken by Munk and his associates (Munk, 2006; Munk and Pryor, 2002; Munk et al, 2003) and is related to several U.S. patents (U.S. Pat. Nos. 6,512,487 B1, 2003; 6,771,221 B2, 2004; 6,876,336 B2, 2005). The CSA is based on the use of planar dipoles as the array element antennas, having a ground plane spaced less than 0.5-wavelength at the highest operating frequency. Their CSA claims a 10:1 bandwidth, yet has only disclosed scant data to support it. Also, a slot-version of CSA has been pursued by Lee and his associates (J. J. Lee, 2007) with a claim of 4:1 bandwidth.
The FA has been reported by Friedrich and his associates (Friedrich et al, 2001; Pringle et al, 2001), and has a U.S. Pat. No. 6,323,809 B1, 2001. The FA employs a multilayer structure with real-time reconfiguration to realize a set of radiating elements and a ground conducting plane generally spaced ¼-wavelength therefrom for the particular operating frequency of interest. The FA approach relies on design optimization processes to generate an optimum array design. Claiming a wide operating bandwidth much more than 10:1, the FA approach has insufficient supporting data in the open literature. The viability of the technique of a movable ground plane by reconfiguration, as claimed in the FA approach, was questioned categorically by Munk and Pryor (2002).
Indeed, as observed by Thors et al (2005), design guidelines and results are often scant or nonexistent in the documents on CSA and FA. It must be emphasized that, even though extremely broad bandwidth can be easily designed for the case of planar arrays of 2-D elements with no supporting ground plane, design of broadband planar array having a ground plane is difficult. This is particularly true in the case of the FA approach, for which Thors et al only managed to achieve a bandwidth of 2.23:1.
This inventor noted that the theory and experimentation on CSA and FA disclosed to the public often are indirect and incomplete, and have not yet realized full-fledged broadband performance as claimed. He also noted some limitations and deficiencies in certain design concepts of CSA and FA, which consist of inherently narrowband components whose bandwidths are difficult to broaden by reconfiguration or optimization. He then conceived the present invention based on the traveling-wave (TW) antenna concept, which potentially has superior performance over prior-art approaches.